A wind turbine is an energy conversion system which converts kinetic wind energy into electrical energy for utility power grids. Specifically, wind is applied to wind turbine blades of the wind turbine to rotate a rotor. The mechanical energy of the rotating rotor in turn is converted into electrical energy by an electrical generator. Because wind speed fluctuates, the force applied to the wind blades and hence the rotational speed of the rotor can vary. Power grids however require a constant frequency electrical power to be provided by the wind turbine.
One type of wind turbine that provides constant frequency electrical power is a fixed-speed wind turbine. This type of wind turbine requires a generator rotor that rotates at a constant speed. A disadvantage of such fixed-speed wind turbine is that it does not harness all of the wind's energy at high speeds and must be disabled at low wind speeds. Another type of wind turbine is a variable speed wind turbine. This type of wind turbine allows the generator to rotate at variable speeds to accommodate for fluctuating wind speeds. By varying the rotating speed of the generator rotor, energy conversion can be optimized over a broader range of wind speeds.
A variable speed wind turbine usually includes a power converter having a generator side converter coupled to a grid side converter via a direct current (DC) link. The generator side converter regulates the power of the generator. This power passes through the DC-link, and is eventually fed to the grid through the grid side converter. The same is true for the Doubly Fed Induction Generator (DFIG) systems where only a portion of the power from the generator passes through the power converter.
Under normal conditions, the electrical power or energy from the generator is supplied to the grid through the power converter. In other words, the energy captured from the wind by the wind turbine is passed to the grid. Therefore, it can be said that there is power balance during normal conditions. However, when there is a sudden wind gust and/or grid fault, this power balance may be disrupted, resulting in more power being generated than power being supplied to the grid. Such power imbalance might lead to undesired tower oscillations, drive train damage or turbine tripping.
Specifically, the power output of the generator in response to a sudden wind gust can be approximated as ramp input to the power system in the wind turbine with a steep slope. Such load ramping is one of the most difficult load behaviors for a control system in the wind turbine. A wind turbine normally handles wind gust by pitching the blades to reduce the speed of rotor as disclosed, for example, in US 2009/0224542 and EP 2107236. However, due to the dynamics of a pitch controller, the pitching of the blade may not be fast enough to respond to the sudden wind gust. Hence this results in the sudden increase in the power generated by the generator, leading to the undesired tower oscillations, etc as mentioned above.
When there is a grid fault, for example a low voltage event, there is a sudden drop in demand for active power from the grid. Since the pitching of the blades is not able to respond fast enough to reduce power generation, there is an imbalance of power in the wind turbine. U.S. Pat. No. 7,411,309 discloses the use of a crowbar circuit during low voltage events at the grid. The crowbar circuit is coupled to the DC link between the generator side converter and the grid side converter. When the DC link voltage exceeds a predetermined value (due to grid fault), the crowbar circuit is activated to drain the excess generator power, hence lowering the DC link voltage.
The use of a crowbar circuit or a dump load circuit may provide a good way of dissipating excess power during a power imbalance event. The dump load circuit is activated by detecting an abnormal increase in the DC link voltage or a sudden drop in grid voltage. However, it may not be the most effective method to handle power imbalance events such as wind gust, or in extreme conditions when wind gust and grid fault happen at the same time. Moreover in this method, the resistor bank in the dump load circuit is excessively stressed.
It is thus an object of the invention to provide an improved way of managing excess power generated in the wind turbine in power imbalance event.